When Microsoft created Windows, it’s vision was of a system that is the ultimate in “cool” (your version of cool may not be the same as the computer programmer geek’s version of cool). Something that would wink and blink at you, and impress you with every thing that you do within the computer.
Therefore, the mouse has a shadow, folders show the thumbnails of their contents, menus are animated to look like they slide out and open, opening a folder is animated to look like the windows “grow” open, etc.
All of these items take work for the computer to process and show on the screen, but a new computer handles them effortlessly. As the computer gets older, and these efforts are needed for the real computer use, we need to turn these off (what? and lose my cool effects? yes, at this point you probably aren’t experiencing the full advantage of the effects due to the slow computer).